Electronics systems, such as servers, typically include a number of printed circuit boards placed into a card cage or rack. A backplane is used to provide electrical interconnection between the circuit boards, and the circuit boards include connectors that mate into corresponding connectors of the backplane. Data can accordingly be transferred between circuit boards over the backplane via electrical signals.
As data bandwidths between circuit cards have increased, many difficulties with providing data transfer over a backplane have arisen. As data rates are increased, electromagnetic interference and crosstalk can become a problem, making the backplane design more difficult and expensive. Non-ideal characteristics presented by electrical connectors also prove increasingly problematic as data rates are increased.
Increasing numbers of parallel interconnections can be used to provide bandwidth increases, but this leads to increased pin and component counts as well as higher power consumption. Very large backplane connectors can also result in high insertion forces and reduced reliability.
Sometimes, auxiliary connections between boards have been used in addition to the backplane connections, including for example ribbon cables between connectors disposed on board front edges (the edges opposite the backplane). Of course, these auxiliary connections also face similar electrical design challenges as the backplane with respect to signal quality, electromagnetic interference, and other problems.
Increasingly, systems designers are turning to optical interconnect as an alternative to conventional electronic interconnects. Optical interconnects provide the potential for very high bandwidths while providing generally less electromagnetic interference generation and susceptibility than electrical interconnects.
While optical backplanes have been proposed, optical backplanes tend to be complex and expensive. In part, expense results from the difficulty of providing alignment of optical inputs and outputs from the boards to the backplane. In part, this difficulty results from relatively low tolerance mechanical structures used in card cages. While electrical connectors are designed to accommodate these tolerances, providing the higher alignment desired in optical connectors can add significant cost and mechanical complexity to the card cage and connectors.
Optical interconnection can use optical cables connected between boards. Unfortunately, large numbers of optical cables are often used to provide interconnections between many different pairs of circuit boards adding costs to the system.